Polymorphism of Tla genes and TL products is further defined by new TL monoclonal antibodies. One of these has features that may point to a coding sequence concerned in normal regulation of TL expression. A previously unknown TL product of high molecular weight and peculiar to the Tlad and Tlae genotypes is being investigated for its relation to the normal TL heavy chain. The products of several different Tla alleles are also in various degrees distinguishable by their isoelectric points. The TL heavy chains relative to a given Tla type appear characteristically to assume two forms with a slight difference in mobility, and the reasons for this are being studied. Since low incorporation of label has been a main obstacle to conventional microsequencing of TL, much attention has been devoted to preparative methods that will yield sufficient quantities of labeled CNBr fragments. The protocols now described should be sufficient to overcome these difficulties, and modified procedures that could lead to gas phase sequencing (in collaboration) as an additional method are under study. The use of technology for studying Tla genes is now a virtually indispensable adjunct to investigation of TL product structure. The emphasis of this aspect of the work outlined is on the derivation of Tla-selective probes in relation especially to the critical prototype Tlaa haplotype of strain A, on which our program centers.